1. Field of the Invention
The present invention relates to magnetic iron oxides suitable for use in magnetic recording media such as magnetic tapes, magnetic discs, magnetic sheets, etc. More particularly, the present invention relates to magnetic iron oxides having a low number of pores per unit area, high coercive force, and excellent dispersibility and orientation.
2. Description of the Prior Art
In the preparation of high density magnetic recording media, magnetic materials having high coercive force, excellent acicular properties, excellent squareness ratio and good dispersibility are required. As is well known in this art, ferromagnetic iron oxides are obtained by the dehydration of iron oxyhydroxides (.alpha.-FeOOH (goethite), .beta.-FeOOH, .gamma.-FeOOH (lepidocrocite)), reduction and oxidation, in this order.
Typically, goethite is first converted into .alpha.-Fe.sub.2 O.sub.3 by dehydration; the resulting .alpha.-Fe.sub.2 O.sub.3 is reduced to form magnetite (Fe.sub.2 O.sub.3); the magnetite is finally converted into maghemite (.gamma.-Fe.sub.2 O.sub.3 : ferromagnetic iron oxide). This technique is described in Japanese Patent Publications Nos. 7776/51, 3292/56, 14090/69, 25959/72, and 39477/72; Japanese Patent Application Laid Open Nos. 40097/72 and 15699/74; and U.S. Pat. Nos. 2,127,907 and 2,388,659.
In order to improve the coercive force of iron oxide type magnetic powders, it is effective to dope Co therein. This technique is described in U.S. Pat. Nos. 3,117,333 and 3,671,435; Japanese Patent Publications Nos. 6538/66, 27719/66, 6113/67, 10994/73, 15759/73 and 4264/74; and Japanese Patent Application Laid Open Nos. 22702/72, 1998/73, 76097/73, 87397/73 and 101599/73.
However, Co-containing ferromagnetic iron oxides have such faults as pressure demagnetization, demagnetization by heat, the coercive force changes with the passage of time, an uneven coercive force distribution, and deterioration in erasure characteristics and then print-through ratio. Thus, improvement of Co-containing ferromagnetic iron oxides at these points has been desired.
In addition, there are many pores in ferromagnetic iron oxides as a result of the removal of water during dehydration as above described. The presence of pores results in an apparent decrease of magnetization per unit volume of the ferromagnetic iron oxides and also results in magnetic domains within a particle of the ferromagnetic iron oxide, causing aggregation of ferromagnetic iron oxides in a magnetic layer. Further, orientation which should be performed in the preparation of magnetic recording media is difficult, so that magnetic recording characteristics are deteriorated.
In order to minimize the formation of pores, several methods have been proposed. These methods include covering the surface of iron oxyhydroxides with inorganic materials or organic materials as are hereinafter illustrated prior to dehydration, reduction and oxidation; the adding of specific ions thereto; and using carefully controlled conditions for the dehydration, reduction and oxidation. Unless such special methods are used, a heating at a high temperature is required in the dehydration, reduction and oxidation steps in order to obtain ferromagnetic materials having minimal pores and a high coercive force, e.g., heating to about 500.degree. to about 800.degree. C for .alpha.-Fe.sub.2 O.sub.3 and about 300.degree. to about 500.degree. C for Fe.sub.2 O.sub.3. Such heating, however, promotes undesired sintering or ferromagnetic iron oxides, and it is difficult to disperse such ferromagnetic iron oxides in a binder when sintering is excessive and the physical properties of the ferromagnetic iron oxide surface change.
Although it takes a long period of time for a complete dispersion as described above, it is extremely difficult by established methods to obtain ferromagnetic powders having less pores from iron oxyhydroxides since all prior art production methods by necessity include a dehydration step. The present invention overcomes these shortcomings of the prior art.